1. Field of the Invention
The present invention relates to an automatic welding apparatus for performing an upward electroslag or electrogas arc welding of a structure having curved joints such as arcuate ones, and to a welding controlling method for operating the welding apparatus.
2. Description of the Prior Art
End plates are incorporated in reaction towers and other pressure vessels. The end plates can be easily manufactured unitarily, by means of a fullering making use of a press, if the size of the destined end plate is small enough. However, the end plate of a large size reaction tower or other pressure vessel can hardly be produced unitarily. Thus, conventionally, end plates of large size have been produced by assembling and welding a plurality of sections.
Usually, these sections are a top panel section and a hemispherical section, and the latter is further divided into a plurality of pieces or so-called petals adapted to form, when welded to one another, the hemispherical section of the end plate.
Conventionally, the welding of the petals to form a unitary body has been performed by a method as illustrated in FIG. 1. More specifically, the petal-like pieces to be welded are mounted and fixed on a positioner 1. Then, a plurality of welding nozzles 4, 4 are advanced from an electroslag welder 3 situated at a predetermined position so that the welding nozzles may uniformly cover the groove surfaces. Then, the petal-like pieces are rotated by the positioner, in accordance with the progress of the welding, so as to obtain the molten slag at that position, thus keeping a constant welding posture.
The reason for this rotation of the welded pieces by the positioner, while the welding nozzles are kept stationary, is that the electroslag welding is most conveniently carried out in vertical welding.
However, this conventional method disadvantageously necessitates a specific positioner capable of rotating around the center of the bottom of the hemispherical section. In addition, this method cannot be applied to a conical object. At the same time, when this conventional method is applied to a structure of the aforementioned shape, the scale of the positioner has to be as large as the thickness of the pieces to be welded increases, so that it becomes necessary to uneconomically enlarge the size of the factory house for providing a large space to accomodate the large positioner.
Further, in the conventional method relying upon rotation of the welded pieces by a positioner, for performing the assembling only by vertical welding, there exists a vital drawback in that only one joint can be processed at a time so as to make the efficiency of the work extremely slow. Moreover, a large strain is caused in the final assembly, which can be recovered only through a troublesome step of straightening.